Provides an instance of Arrow for process pipelines. As described in
the original paper on arrows, they can be used to represent stream processing,
so CHP seemed like a possible fit for an arrow.

Whether this is actually an instance of Arrow depends on technicalities.
This can be demonstrated with the arrow law arr id >>> f = f = f >>> arr
id. Whether CHP satisfies this arrow law depends on the definition of
equality.

If equality means that given the same input value, both arrows produce the
same corresponding output value, this is an arrow.

If equality means you give the arrows the same single input and wait for the single output,
and the output is the same, this is an arrow.

If equality means that you can feed the arrows lots of inputs (one after
the other) and the behaviour should be the same with regards to communication,
this is not an arrow.

The problem lies in the buffering inherent in arrows. Imagine if f is
a single function. f is effectively a buffer of one. You can feed it
a single value, but no more than that until you read its output. However,
if you have arr id >>> f, that can accept two inputs (one held by the
arr id process and one held by f) before you must accept the output.

I am fairly confident that the arrow laws are satisfied for the
definition of equality that given the same single input, they will
produce the same single output. If you don't worry too much about the
behavioural difference, and just take arrows as another way to wire
together a certain class of process network, you should do fine.

Given a ProcessPipeline (formed using its Arrow instance) and
the channels to plug into the ends of the pipeline, returns the process
representing the pipeline.

The pipeline will run forever (until poisoned) and you must run it in
parallel to whatever is feeding it the inputs and reading off the outputs.
Imagine that you want a process pipeline that takes in a pair of numbers,
doubles the first and adds one to the second. You could encode this
in an arrow using:

runPipeline (arr (*2) *** arr (+1))

Arrows are more useful where you already have processes written that
process data and you want to easily wire them together. The arrow notation
is probably easier for doing that than declaring all the channels yourself
and composing everything in parallel.

Adds a wrapper that forms this process into the right data type to be
part of an arrow.

Any process you apply this to should produce exactly one output per
input, or else you will find odd behaviour resulting (including deadlock).
So for example, don't use arrowProcess (Control.Concurrent.CHP.Common.filter
...) or arrowProcess Control.Concurrent.CHP.Common.stream inside any arrow combinators
other than >>> and <<<.

Like the arr function of the ProcessPipeline arrow instance, but fully evaluates
the result before sending it. If you are building process pipelines with arrows to
try and get some parallel speed-up, you should try this function instead of
arr itself.

ProcessPipelineLabel is a version of ProcessPipeline that allows the processes
to be labelled, and thus in turn for the channels connecting the processes to
be automatically labelled. ProcessPipelineLabel is not an instance of Arrow,
but it does have a lot of similarly named functions for working with it. This
awkwardness is due to the extra Show constraints on the connectors that allow
the arrow's contents to appear in traces.

If you don't use traces, use ProcessPipeline. If you do use traces, and want
to have better labels on the process and values used in your arrows, consider
switching to ProcessPipelineLabel.

ProcessPipelineLabel and all the functions that use it, were added in version
1.5.0.